In these days, a semiconductor laser in which oscillation is performed with the single axis mode and the high spectrum purity can be utilized because the characteristic of the semiconductor laser is improved. Therefore, a coherent optical communication system in which frequency and phase of light are used for transmitting informations can be realized with the high sensitivity. In a FSK modulation system in which frequency of light is varied dependent on informations, especially, long distance transmission can be realized without any repeater because the insertion loss of an external modulator is avoided for the reason why modulated light is easily obtained in accordance with the direct modulation of light in which signal current of low levels is added to bias current of the semiconductor laser. In the FSK modulation system, transmitting signals are reproduced in accordance with the demodulation of phase informations based on frequency informations by use of a delay detection circuit etc. in which the continuous phase characteristic of frequency modulated light is utilized because the frequency modulated light obtained from the semiconductor laser directly modulated has the continuous-phase characteristic. According to such a FSK modulation system, a sensitivity which is substantially equal to that of a phase shift keying modulation system having the highest sensitivity among the various kinds of modulation systems.
In the FSK modulation system, the frequency deviation can be decreased down to a half of transmission rate which is the minimum frequency deviation necessary for the independency of signals. Therefore, the modulation spectrum band can be narrowered. Especially, research and development have been intensively promoted in a binary continuous phase FSK system which is suitable for a high speed modulation system as reported in "The experiment of 2 Gb/s optical heterodyne transmission" described in "The Technical Digest of the Institute of Electronics, Information and Communication Engineers, 1987, 70 year Anniversary National Convention Record, part 10, 2369."
On the other hand, a multiple level code communication system is an advantageous system by which a large capacity of data are transmitted in a communication system having a limited transmission capacity due to response speed of devices included therein. In such a multiple level code communication system, informations can be transmitted in capacity as much as twice in the same modulation rate in a case where, for instance, two binary codes are converted into a four level code. For this reason, the transmission capacity becomes twice by using a device of a response speed which is the same as that required in a binary code transmission.
According to the binary continuous phase FSK modulation system, however, the possibility of being applied to a detection system in a multiple level continuous phase FSK modulation system has not been studied. Therefore, a semiconductor device must be developed to increase the response speed in frequency modulation in a case where a high speed continuous phase FSK modulation system using a semiconductor laser is required to be put into a practical use.